An offender may be fitted with an electronic tag in order to enforce conditions prescribed to the offender as a punishment for an offence, as a condition of their release on bail or as a condition of their early release from prison. One of the conditions that can be enforced by an electronic tag is a curfew, which prevents the offender leaving their home at certain times, especially at night.
Electronic tags typically emit a radio-frequency communication which identifies the wearer of the tag. A monitoring unit is located in the offender's home. The radio-frequency communication from the electronic tag is received by the monitoring unit whenever the electronic tag is in the vicinity of the monitoring unit. If the monitoring unit does not receive the radio-frequency communication, this indicates that the offender has moved out of range of the monitoring unit, for example, by leaving their home.
When the offender is under a curfew, the offender should not leave their home between certain times, such as overnight. During the curfew, the monitoring unit checks that the radio-frequency communications are being received from the electronic tag indicating that the offender is still in the vicinity of the monitoring unit, which is taken to mean that the offender is still at home respecting their curfew.
If, when the offender is under curfew, the monitoring unit does not receive, or stops receiving, the radio-frequency communication from the electronic tag or if the signal strength of the radio-frequency communication drops below a signal strength threshold, this indicates that the offender is no longer in the vicinity of the monitoring unit which means that the offender has probably left their home and is breaking their curfew. The monitoring unit sends an alarm communication to a monitoring centre alerting the monitoring centre to the possibility that the offender has broken their curfew.
Offenders usually have a strong desire to break their curfew without detection. It would, therefore, be desirable to find a way to detect whether an offender is breaking, or at least attempting to break, their curfew.